Error message

Deprecated function: The each() function is deprecated. This message will be suppressed on further calls in _menu_load_objects() (line 579 of /home/equsorg/public_sqd/includes/menu.inc).

Quantum hybrid systems with superconducting circuits

Tagged with Quantum hybrid systems

Quantum hybrid systems is a popular modern research field where scientist try to combine quantum systems of different architectures. Superconducting circuits thanks to their strong non-linearities and advance quantum control can be a valuable resource to extend functionality of many other quantum devices. On the other hand superconducting circuits lack the coherence times of their spin and atomic quantum counterparts.

Different systems can be incorporated into superconducting circuits to achieve a particular goal. For example, coupling supercondcuting circuits to a nanomechnical oscillator was the most successful approach to cool down the mechanical degree of freedom to its ground state. There have also been a number of successful experiments where strong couling regime between superconducting resonator or a qubit with an ensemble of spins were achieved opening a path to realize a superconducting quantum processor with a spin-based quantum memory.

As part of EQuS Centre our team has a unique opportunity to get in touch with Australian leading researchersworking on other quantum systems. We are currenly collaborating with Prof. Michael Tobar on coupling a superconducting qubit to a macroscopic acoustic cavity.

The ARC Centre of Excellence for Engineered Quantum Systems

This Centre of Excellence seeks to initiate the Quantum Era in the 21st century by engineering designer quantum systems.

Neural networks for on-the-fly single-shot state classification
Navarathna Rohit et al, 2021
Applied Physics Letters

Neural networks have proven to be efficient for a number of practical applications ranging from image recognition to identifying phase transitions in quantum physics models.

Measuring effective temperatures of qubits using correlations
Kulikov A, Navarathna R and Fedorov A, 2020
Phys. Rev. Lett., 124, pp. 240501

Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. Qubits are commonly initialized by cooling to their ground states through passive thermalization or by using active reset protocols.

Probabilistic motional averaging
Karpov D et al, 2020
EPJ B, 93, pp. 49

In a continuous measurement scheme a spin-1/2 particle can be measured and simultaneously driven by an external resonant signal.

Quantum rifling: protecting a qubit from measurement back-action
Szombati D. et al, 2020
Phys. Rev. Lett, 124, pp. 70401

Quantum mechanics postulates that measuring the qubit's wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate the qubit collapsed into. We show this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the time-averaged expectation value of the measurement operator, erasing information about the initial state of the qubit, while completely suppressing the measurement back-action. We call this regime `quantum rifling', as the fast spinning of the Bloch vector protects it from deflection into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to the same probe field and demonstrate that quantum rifling allows us to measure either one of the two qubits on demand while protecting the state of the other from measurement back-action. Our results allow for the implementation of selective read out multiplexing of several qubits, contributing to efficient scaling up of quantum processors for future quantum technologies.

In situ characterization of qubit control lines: a qubit as a vector network analyzer
Jerger M. et al, 2019
Phys. Rev. Lett., 123, pp. 150501

We present a technique to measure the transfer function of a control line using a qubit as a vector network analyzer. Our method requires coupling the line under test to the the longitudinal component of the Hamiltonian of the qubit and the ability to induce Rabi oscillations through simultaneous driving of the transversal component. We used this technique to characterize the 'flux' control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our method can be used for the qubit 'flux' line calibration to increase the fidelity of entangling gates for the quantum processor. The qubit can be also used as a microscopic probe of the electro-magnetic fields on a chip. 

Nonreciprocity Realized with Quantum Nonlinearity
Hamann A.R. et al, 2018
Phys. Rev. Lett., 121, pp. 123601

Our experiment comprises the first experimental implemenation of a so called "optical rectifier" or a non-reciprocal device based on quantum nonlinearity of two-level atoms. Our device is also the simplest possible device which can behave non-reciprocally and shows the connection between non-reciprocty and entangelement.

Nonreciprocal Atomic Scattering: A saturable, quantum Yagi-Uda antenna
Müller C. et al, 2017
Phys. Rev. A, 96, pp. 53817

Recent theoretical studies of a pair of atoms in a 1D waveguide find that the system responds asymmetrically to incident fields from opposing directions at low powers. Since there is no explicit time-reversal symmetry breaking elements in the device, this has caused some debate. Here we show that the asymmetry arises from the formation of a quasi-dark-state of the two atoms, which saturates at extremely low power. In this case the nonlinear saturability explicitly breaks the assumptions of the Lorentz reciprocity theorem. Moreover, we show that the statistics of the output field from the driven system can be explained by a very simple stochastic mirror model and that at steady state, the two atoms and the local field are driven to an entangled, tripartite |W⟩ state. Because of this, we argue that the device is better understood as a saturable Yagi-Uda antenna, a distributed system of differentially-tuned dipoles that couples asymmetrically to external fields. 

Realization of a quantum random generator certified with the Kochen-Specker theorem
Kulikov A. et al, 2017
Physical Review Letters, 119, pp. 240501

We experimentally realize a quantum random number generator (QRNG) with randomness of each generated number certified by the Kochen-Specker theorem. Employing this novel certification scheme eliminates the necessity for input seed random numbers and lifts the non-locality requirement for the certified QRNG. Implementation on the base of superconducting qubits allows achieving a bit rate of 25 kBit/s, two orders of magnitude higher than previously reported certified QRNGs. These improvements make our result a major step towards realization a practical certified quantum QRNG with implications for digital security and modelling algorithms.

Realization of a binary-outcome projection measurement of a three-level superconducting quantum system
Jerger M. et al, 2016
Phys. Rev. Appl., 6, pp. 14014

The ability to determine whether a multi-level quantum system is in a certain state while preserving quantum coherence between all orthorgonal states is necessary to realize binary-outcome compatible measurements which are, in turn, a prerequisite for testing the contextuality of quantum mechanics. In this paper, we use a three-level superconducting system (a qutrit) coupled to a microwave cavity to explore different regimes of quantum measurement. In particular, we engineer the dispersive shifts of the cavity frequency to be identical for the first and second excited states of the qutrit which allows us to realize a strong projective binary-outcome measurement onto its ground state with a fidelity of 94.3%. Complemented with standard microwave control and low-noise parametric amplification, this scheme can be used to create sets of compatible measurements to reveal the contextual nature of superconducting circuits. 

3D microwave cavity with magnetic flux control and enhanced quality factor
Reshitnyk Y., Jerger M. and Fedorov A., 2016
EPJ Quantum Technol., 3, 1, pp. 1-6

Three-dimensional (3D) microwave cavities with embedded superconducting quantum bits (qubits), provide a popular and versatile platform for quantum information processing and hybrid quantum systems. A current issue that has arisen is that 3D superconducting cavities do not permit magnetic field control of qubits embedded in these cavities. In contrast, microwave cavities made of normal metals can be transparent to magnetic fields, but experience a much lower quality factor, which negates many of the advantages of the 3D architecture. Here we presented measurements of a device that bridges a gap between these two types of cavities with magnetic field control and an order of magnitude higher quality factor compared to all previously tested copper cavities. An added benefit to that our hybrid cavity possesses is that it also provides an improved thermal link to the sample that superconducting cavities alone cannot provide. A large improvement in quality factor and magnetic field control makes this 3D hybrid cavity an attractive new platform for circuit quantum electrodynamics experiments.

Pages